Field of the Invention
Embodiments of the present invention relate to a multi charged particle beam writing apparatus and a multi charged particle beam writing method, and, for example, to a method for correcting an error of a gray-scale value based on which the irradiation time is controlled in multi beam writing.
Description of Related Art
The lithography technique that advances miniaturization of semiconductor devices is extremely important as a unique process whereby patterns are formed in semiconductor manufacturing. In recent years, with high integration of LSI, the line width (critical dimension) required for semiconductor device circuits becomes progressively narrower year by year. The electron beam writing technique which intrinsically has excellent resolution is used for writing or “drawing” a mask pattern on a mask blank with electron beams.
For example, as a known example of employing the electron beam writing technique, there is a writing apparatus using multi-beams. Since it is possible for multi-beam writing to irradiate multiple beams at a time, the writing throughput can be greatly increased in comparison with single electron beam writing. For example, a writing apparatus employing the multi-beam technique forms multi-beams by letting portions of an electron beam emitted from an electron gun pass through a corresponding hole of a plurality of holes in a mask, performs blanking control for each beam, reduces each unblocked beam by an optical system in order to reduce a mask image, and deflects the beam by a deflector so as to irradiate a desired position on a target object or “sample”.
In multi-beam writing, the dose of each beam is controlled by irradiation time. The irradiation time is defined by n-bit gray-scale value data. Accordingly, the irradiation time can be controlled by a control signal of 0 to (2n−1) gray scale level of gradation by gray scale levels. As the number of bits of the control signal increases, the data volume also increases due to the increased number of bits. In multi-beam writing, since irradiation of many beams is performed at a time, it is necessary to transmit a lot of irradiation time data corresponding to the number of beams in a short time. Thus, writing time is greatly affected by the time needed for transmitting data. Therefore, the data volume of irradiation time data is preferably as small as possible.
On the other hand, if performing dose correction, such as proximity effect correction, the range of dose modulation for a dose applied onto each irradiation position needs to be several hundred percent, for example, compared with respect to a base dose. For example, in the case of defining the irradiation time corresponding to a dose by gray-scale values of 0 to L gray level, there is a gray scale level error per gray level due to gradation by gray scale levels, where the error is (hundreds/L) % at the maximum. With recent micropatterning and increase in accuracy, pattern dimension variation associated with such error of gradation by gray scale levels is not negligible. If increasing the maximum gray-scale value L, the resolution can be increased and an error of gradation by gray scale levels can be reduced. However, as described above, data volume increases due to the increased maximum gray-scale value L, thus resulting in a problem in that the writing time becomes long.
Although not being a method for correcting errors of gradation, by gray scale levels, irradiation time, it is proposed, when fractional values (digits after the decimal point) are generated in representing a desired dose by integer values, to add an integer value close to the sum of fractional values of each pixel in a group to a central pixel in the group (e.g., refer to Japanese Patent Application Laid-open (JP-A) No. 2016-058714).
Critical dimension (line width) CD of a pattern to be written can be controlled by dose modulation of a pixel located close to the edge of the pattern. Therefore, if an error of gradation by gray scale levels occurs in the irradiation time to irradiate a pixel located close to the edge, the dimension deviates (shifts) due to the error of the gradation by gray scale levels.